Astronomy & Astrophysics manuscript no. DR2HRD c ESO 2018 August 14, 2018
Gaia Data Release 2: Observational Hertzsprung-Russell diagrams ?
Gaia Collaboration, C. Babusiaux1, 2, F. van Leeuwen3, M.A. Barstow4, C. Jordi5, A. Vallenari6, D. Bossini6, A. Bressan7, T. Cantat-Gaudin6, 5, M. van Leeuwen3, A.G.A. Brown8, T. Prusti9, J.H.J. de Bruijne9, C.A.L. Bailer-Jones10, M. Biermann11, D.W. Evans3, L. Eyer12, F. Jansen13, S.A. Klioner14, U. Lammers15, L. Lindegren16, X. Luri5, F. Mignard17, C. Panem18, D. Pourbaix19, 20, S. Randich21, P. Sartoretti2, H.I. Siddiqui22, C. Soubiran23, N.A. Walton3, F. Arenou2, U. Bastian11, M. Cropper24, R. Drimmel25, D. Katz2, M.G. Lattanzi25, J. Bakker15, C. Cacciari26, J. Castañeda5, L. Chaoul18, N. Cheek27, F. De Angeli3, C. Fabricius5, R. Guerra15, B. Holl12, E. Masana5, R. Messineo28, N. Mowlavi12, K. Nienartowicz29, P. Panuzzo2, J. Portell5, M. Riello3, G.M. Seabroke24, P. Tanga17, F. Thévenin17, G. Gracia-Abril30, 11, G. Comoretto22, M. Garcia-Reinaldos15, D. Teyssier22, M. Altmann11, 31, R. Andrae10, M. Audard12, I. Bellas-Velidis32, K. Benson24, J. Berthier33, R. Blomme34, P. Burgess3, G. Busso3, B. Carry17, 33, A. Cellino25, G. Clementini26, M. Clotet5, O. Creevey17, M. Davidson35, J. De Ridder36, L. Delchambre37, A. Dell’Oro21, C. Ducourant23, J. Fernández-Hernández38, M. Fouesneau10, Y. Frémat34, L. Galluccio17, M. García-Torres39, J. González-Núñez27, 40, J.J. González-Vidal5, E. Gosset37, 20, L.P. Guy29, 41, J.-L. Halbwachs42, N.C. Hambly35, D.L. Harrison3, 43, J. Hernández15, D. Hestroffer33, S.T. Hodgkin3, A. Hutton44, G. Jasniewicz45, A. Jean-Antoine-Piccolo18, S. Jordan11, A.J. Korn46, A. Krone-Martins47, A.C. Lanzafame48, 49, T. Lebzelter50, W. Löffler11, M. Manteiga51, 52, P.M. Marrese53, 54, J.M. Martín-Fleitas44, A. Moitinho47, A. Mora44, K. Muinonen55, 56, J. Osinde57, E. Pancino21, 54, T. Pauwels34, J.-M. Petit58, A. Recio-Blanco17, P.J. Richards59, L. Rimoldini29, A.C. Robin58, L.M. Sarro60, C. Siopis19, M. Smith24, A. Sozzetti25, M. Süveges10, J. Torra5, W. van Reeven44, U. Abbas25, A. Abreu Aramburu61, S. Accart62, C. Aerts36, 63, G. Altavilla53, 54, 26, M.A. Álvarez51, R. Alvarez15, J. Alves50, R.I. Anderson64, 12, A.H. Andrei65, 66, 31, E. Anglada Varela38, E. Antiche5, T. Antoja9, 5, B. Arcay51, T.L. Astraatmadja10, 67, N. Bach44, S.G. Baker24, L. Balaguer-Núñez5, P. Balm22, C. Barache31, C. Barata47, D. Barbato68, 25, F. Barblan12, P.S. Barklem46, D. Barrado69, M. Barros47, S. Bartholomé Muñoz5, J.-L. Bassilana62, U. Becciani49, M. Bellazzini26, A. Berihuete70, S. Bertone25, 31, 71, L. Bianchi72, O. Bienaymé42, S. Blanco-Cuaresma12, 23, 73, T. Boch42, C. Boeche6, A. Bombrun74, R. Borrachero5, S. Bouquillon31, G. Bourda23, A. Bragaglia26, L. Bramante28, M.A. Breddels75, N. Brouillet23, T. Brüsemeister11, E. Brugaletta49, B. Bucciarelli25, A. Burlacu18, D. Busonero25, A.G. Butkevich14, R. Buzzi25, E. Caffau2, R. Cancelliere76, G. Cannizzaro77, 63, R. Carballo78, T. Carlucci31, J.M. Carrasco5, L. Casamiquela5, M. Castellani53, A. Castro-Ginard5, P. Charlot23, L. Chemin79, A. Chiavassa17, G. Cocozza26, G. Costigan8, S. Cowell3, F. Crifo2, M. Crosta25, C. Crowley74, J. Cuypers†34, C. Dafonte51, Y. Damerdji37, 80, A. Dapergolas32, P. David33, M. David81, P. de Laverny17, F. De Luise82, R. De March28, D. de Martino83, R. de Souza84, A. de Torres74, J. Debosscher36, E. del Pozo44, M. Delbo17, A. Delgado3, H.E. Delgado60, S. Diakite58, C. Diener3, E. Distefano49, C. Dolding24, P. Drazinos85, J. Durán57, B. Edvardsson46, H. Enke86, K. Eriksson46, P. Esquej87, G. Eynard Bontemps18, C. Fabre88, M. Fabrizio53, 54, S. Faigler89, A.J. Falcão90, M. Farràs Casas5, L. Federici26, G. Fedorets55, P. Fernique42, F. Figueras5, F. Filippi28, K. Findeisen2, A. Fonti28, E. Fraile87, M. Fraser3, 91, B. Frézouls18, M. Gai25, S. Galleti26, D. Garabato51, F. García-Sedano60, A. Garofalo92, 26, N. Garralda5, A. Gavel46, P. Gavras2, 32, 85, J. Gerssen86, R. Geyer14, P. Giacobbe25, G. Gilmore3, S. Girona93, G. Giuffrida54, 53, F. Glass12, M. Gomes47, M. Granvik55, 94, A. Gueguen2, 95, A. Guerrier62, J. Guiraud18, R. 22 2 85, 32 11, 10 2 46 75 arXiv:1804.09378v2 [astro-ph.SR] 13 Aug 2018 Gutiérrez-Sánchez , R. Haigron , D. Hatzidimitriou , M. Hauser , M. Haywood , U. Heiter , A. Helmi , J. Heu2, T. Hilger14, D. Hobbs16, W. Hofmann11, G. Holland3, H.E. Huckle24, A. Hypki8, 96, V. Icardi28, K. Janßen86, G. Jevardat de Fombelle29, P.G. Jonker77, 63, Á.L. Juhász97, 98, F. Julbe5, A. Karampelas85, 99, A. Kewley3, J. Klar86, A. Kochoska100, 101, R. Kohley15, K. Kolenberg102, 36, 73, M. Kontizas85, E. Kontizas32, S.E. Koposov3, 103, G. Kordopatis17, Z. Kostrzewa-Rutkowska77, 63, P. Koubsky104, S. Lambert31, A.F. Lanza49, Y. Lasne62, J.-B. Lavigne62, Y. Le Fustec105, C. Le Poncin-Lafitte31, Y. Lebreton2, 106, S. Leccia83, N. Leclerc2, I. Lecoeur-Taibi29, H. Lenhardt11, F. Leroux62, S. Liao25, 107, 108, E. Licata72, H.E.P. Lindstrøm109, 110, T.A. Lister111, E. Livanou85, A. Lobel34, M. López69, S. Managau62, R.G. Mann35, G. Mantelet11, O. Marchal2, J.M. Marchant112, M. Marconi83, S. Marinoni53, 54, G. Marschalkó97, 113, D.J. Marshall114, M. Martino28, G. Marton97, N. Mary62, D. Massari75, G. Matijevicˇ86, T. Mazeh89, P.J. McMillan16, S. Messina49, D. Michalik16, N.R. Millar3, D. Molina5, R. Molinaro83,
Article number, page 1 of 28 A&A proofs: manuscript no. DR2HRD
L. Molnár97, P. Montegriffo26, R. Mor5, R. Morbidelli25, T. Morel37, D. Morris35, A.F. Mulone28, T. Muraveva26, I. Musella83, G. Nelemans63, 36, L. Nicastro26, L. Noval62, W. O’Mullane15, 41, C. Ordénovic17, D. Ordóñez-Blanco29, P. Osborne3, C. Pagani4, I. Pagano49, F. Pailler18, H. Palacin62, L. Palaversa3, 12, A. Panahi89, M. Pawlak115, 116, A.M. Piersimoni82, F.-X. Pineau42, E. Plachy97, G. Plum2, E. Poggio68, 25, E. Poujoulet117, A. Prša101, L. Pulone53, E. Racero27, S. Ragaini26, N. Rambaux33, M. Ramos-Lerate118, S. Regibo36, C. Reylé58, F. Riclet18, V. Ripepi83, A. Riva25, A. Rivard62, G. Rixon3, T. Roegiers119, M. Roelens12, M. Romero-Gómez5, N. Rowell35, F. Royer2, L. Ruiz-Dern2, G. Sadowski19, T. Sagristà Sellés11, J. Sahlmann15, 120, J. Salgado121, E. Salguero38, N. Sanna21, T. Santana-Ros96, M. Sarasso25, H. Savietto122, M. Schultheis17, E. Sciacca49, M. Segol123, J.C. Segovia27, D. Ségransan12, I-C. Shih2, L. Siltala55, 124, A.F. Silva47, R.L. Smart25, K.W. Smith10, E. Solano69, 125, F. Solitro28, R. Sordo6, S. Soria Nieto5, J. Souchay31, A. Spagna25, F. Spoto17, 33, U. Stampa11, I.A. Steele112, H. Steidelmüller14, C.A. Stephenson22, H. Stoev126, F.F. Suess3, J. Surdej37, L. Szabados97, E. Szegedi-Elek97, D. Tapiador127, 128, F. Taris31, G. Tauran62, M.B. Taylor129, R. Teixeira84, D. Terrett59, P. Teyssandier31, W. Thuillot33, A. Titarenko17, F. Torra Clotet130, C. Turon2, A. Ulla131, E. Utrilla44, S. Uzzi28, M. Vaillant62, G. Valentini82, V. Valette18, A. van Elteren8, E. Van Hemelryck34, M. Vaschetto28, A. Vecchiato25, J. Veljanoski75, Y. Viala2, D. Vicente93, S. Vogt119, C. von Essen132, H. Voss5, V. Votruba104, S. Voutsinas35, G. Walmsley18, M. Weiler5, O. Wertz133, T. Wevers3, 63, Ł. Wyrzykowski3, 115, A. Yoldas3, M. Žerjal100, 134, H. Ziaeepour58, J. Zorec135, S. Zschocke14, S. Zucker136, C. Zurbach45, and T. Zwitter100 (Affiliations can be found after the references)
Received ; accepted
ABSTRACT
Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies. Key words. parallaxes – Hertzsprung-Russell and C-M diagrams – solar neighbourhood – Stars: evolution
1. Introduction geneous samples that cover the full HRD. Moreover, a precise HRD provides a great framework for exploring stellar popula- The Hertzsprung-Russell diagram (HRD) is one of the most im- tions and stellar systems. portant tools in stellar studies. It illustrates empirically the rela- tionship between stellar spectral type (or temperature or colour Up to now, the most complete solar neighbourhood empir- index) and luminosity (or absolute magnitude). The position of ical HRD could be obtained by combining the Hipparcos data a star in the HRD is mainly given by its initial mass, chemical (Perryman et al. 1995) with nearby stellar catalogues to provide composition, and age, but effects such as rotation, stellar wind, the faint end (e.g. Gliese & Jahreiß 1991; Henry & Jao 2015). magnetic field, detailed chemical abundance, over-shooting, and Clusters provide empirical HRDs for a range of ages and metal non-local thermal equilibrium also play a role. Therefore, the contents and are therefore widely used in stellar evolution stud- detailed HRD features are important to constrain stellar structure ies. To be conclusive, they need homogeneous photometry for and evolutionary studies as well as stellar atmosphere modelling. inter-comparisons and astrometry for good memberships. Up to now, a proper understanding of the physical process in the With its global census of the whole sky, homogeneous as- stellar interior and the exact contribution of each of the effects trometry, and photometry of unprecedented accuracy, Gaia DR2 mentioned are missing because we lack large precise and homo- is setting a new major step in stellar, galactic, and extragalactic studies. It provides position, trigonometric parallax, and proper ? The full Table A.1 is only available at the CDS via anonymous ftp to motion as well as three broad-band magnitudes (G, GBP, and cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz- GRP) for more than a billion objects brighter than G∼20, plus bin/qcat?J/A+A/616/A10 radial velocity for sources brighter than GRVS∼12 mag and pho-
Article number, page 2 of 28 Gaia Collaboration et al.: Gaia Data Release 2: Observational Hertzsprung-Russell diagrams tometry for variable stars (Gaia Collaboration et al. 2018a). Lindegren et al. (2018) described that a five-parameter so- The amount, exquisite quality, and homogeneity of the data al- lution is accepted only if at least six visibility periods are used lows reaching a level of detail in the HRDs that has never been (e.g. the number of groups of observations separated from other reached before. The number of open clusters with accurate par- groups by a gap of at least four days, the parameter is named allax information is unprecedented, and new open clusters or as- visibility_periods_used in the Gaia archive). The observa- sociations will be discovered. Gaia DR2 provides absolute par- tions need to be well spread out in time to provide reliable five- allax for faint red dwarfs and the faintest white dwarfs for the parameter solutions. Here we applied a stronger filter on this first time. parameter: visibility_periods_used>8. This removes strong This paper is one of the papers accompanying the Gaia outliers, in particular at the faint end of the local HRD (Arenou DR2 release. The following papers describe the data used here: et al. 2018). It also leads to more incompleteness, but this is not Gaia Collaboration et al. (2018a) for an overview, Lindegren an issue for this paper. et al. (2018) for the astrometry, Evans et al. (2018) for the The astrometric excess noise is the extra noise that must be photometry, and Arenou et al. (2018) for the global validation. postulated to explain the scatter of residuals in the astrometric Someone interested in this HRD paper may also be interested solution. When it is high, it either means that the astrometric in the variability in the HRD described in Gaia Collaboration solution has failed and/or that the studied object is in a mul- et al. (2018b), in the first attempt to derive an HRD using tem- tiple system for which the single-star solution is not reliable. peratures and luminosities from the Gaia DR2 data of Andrae Without filtering on the astrometric excess noise, artefacts are et al. (2018), in the kinematics of the globular clusters discussed present in particular between the white dwarf and the main se- in Gaia Collaboration et al. (2018c), and in the field kinematics quence in the Gaia HRDs. Some of those stars are genuine bi- presented in Gaia Collaboration et al. (2018d). naries, but the majority are artefacts (Arenou et al. 2018). To In this paper, Sect. 2 presents a global description of how we still see the imprint of genuine binaries on the HRD while re- built the Gaia HRDs of both field and cluster stars, the filters moving most of the artefacts, we adopted the filter proposed p that we applied, and the handling of the extinction. In Sect. 3 we in Appendix C of Lindegren et al. (2018): χ2/(ν0 − 5) < present our selection of cluster data; the handling of the globu- 1.2 max(1, exp(−0.2(G − 19.5)) with χ2 and ν0 given as lar clusters is detailed in Gaia Collaboration et al. (2018c) and astrometric_chi2_al and astrometric_n_good_obs_al, re- the handling of the open clusters is detailed in Appendix A. Sec- spectively, in the Gaia archive. A similar clean-up of the HRD tion 4 discusses the main structures of the Gaia DR2 HRD. The is obtained by the astrometric_excess_noise<1 criterion, but level of the details of the white dwarf sequence is so new that this is less optimised for the bright stars because of the degrees it leads to a more intense discussion, which we present in a of freedom (DOF) issue (Lindegren et al. 2018, Appendix A). separate Sect. 5. In Sect. 6 we compare clusters with a set of Gaia isochrones. In Sect. 7 we study the variation of the Gaia HRDs We built the HRDs by simply estimating the absolute Gaia M with kinematics. We finally conclude in Sect. 8. magnitude in the G band for individual stars using G = G + 5 + 5 log10($/1000.), with $ the parallax in milliarcseconds (plus the extinction, see next section). This is valid only when the relative precision on the parallax is lower than about 20% 2. Building the Gaia HRDs (Luri et al. 2018). We aim here to examine the fine structures in This paper presents the power of the Gaia DR2 astrometry and the HRD revealed by Gaia and therefore adopt a 10% relative photometry in studying fine structures of the HRD. For this, we precision criterion, which corresponds to an uncertainty on MG selected the most precise data, without trying to reach complete- smaller than 0.22 mag: parallax_over_error>10. ness. In practice, this means selecting the most precise parallax Similarly, we apply filters on the relative flux error on the G, and photometry, but also handling the extinction rigorously. This GBP, and GRP photometry: phot_g_mean_flux_over_error>50 can no longer be neglected with the depth of the Gaia precise (σG < 0.022 mag), phot_rp_mean_flux_over_error>20, and data in this release. phot_bp_mean_flux_over_error>20 (σGXP < 0.054 mag). These criteria may remove variable stars, which are specifically studied in Gaia Collaboration et al. (2018b). 2.1. Data filtering The processing of the photometric data in DR2 has not The Gaia DR2 is unprecedented in both the quality and the quan- treated blends in the windows of the blue and red photome- tity of its astrometric and photometric data. Still, this is an inter- ters (BP and RP). As a consequence, the measured BP and mediate data release without a full implementation of the com- RP fluxes may include the contribution of flux from nearby plexity of the processing for an optimal usage of the data. A sources, the highest impact being in sky areas of high stel- detailed description of the astrometric and photometric features lar density, such as the inner regions of globular clusters, the is given in Lindegren et al. (2018) and Evans et al. (2018), re- Magellanic Clouds, or the Galactic Bulge. During the valida- spectively, and Arenou et al. (2018) provides a global validation tion process, misdeterminations of the local background have of them. Here we highlight the features that are important to be also been identified. In some cases, this background is due taken into account in building Gaia DR2 HRDs and present the to nearby bright sources with long wings of the point spread filters we applied in this paper. function that have not been properly subtracted. In other cases, Concerning the astrometric content (Lindegren et al. 2018), the background has a solar type spectrum, which indicates that the median uncertainty for the bright source (G<14 mag) paral- the modelling of the background flux is not good enough. The lax is 0.03 mas. The systematics are lower than 0.1 mas, and the faint sources are most strongly affected. For details, see Evans parallax zeropoint error is about 0.03 mas. Significant correla- et al. (2018) and Arenou et al. (2018). Here, we have limited tions at small spatial scale between the astrometric parameters our analysis to the sources within the empirically defined locus are also observed. Concerning the photometric content (Evans of the (IBP + IRP)/IG fluxes ratio as a function of GBP − GRP 2 et al. 2018), the precision at G=12 is around 1 mmag in the three colour: phot_bp_rp_excess_factor> 1.0+0.015 (GBP −GRP) and 2 passbands, with systematics at the level of 10 mmag. phot_bp_rp_excess_factor< 1.3 + 0.06 (GBP − GRP) . The Gaia
Article number, page 3 of 28 A&A proofs: manuscript no. DR2HRD
kX = AX/A0: 2 3 kX =c1 + c2(GBP − GRP)0 + c3(GBP − GRP)0 + c4(GBP − GRP)0 2 + c5A0 + c6A0 + c7(GBP − GRP)0A0 . (1) As in Danielski et al. (2018), this formula was fitted on a grid of extinctions convolving the latest Gaia passbands presented in Evans et al. (2018) with Kurucz spectra (Castelli & Kurucz 2003) and the Fitzpatrick & Massa (2007) extinction law for 3500 K< Teff <10000 K by steps of 250 K, 0.01< A0 < 5 mag by steps of 0.01 mag and two surfaces gravities: logg = 2.5 and 4. The resulting coefficients are provided in Table 1. We assume in the following A0 = 3.1E(B − V). Some clusters show high differential extinction across their field, which broadens their colour-magnitude diagrams. These clusters have been discarded from this analysis.
3. Cluster data Star clusters can provide observational isochrones for a range of ages and chemical compositions. Most suitable are clusters with low and uniform reddening values and whose magnitude range is wide, which would limit our sample to the nearest clusters. Fig. 1. Full Gaia colour-magnitude diagram of sources with the filters Such a sample would, however, present a rather limited range in described in Sect. 2.1 applied (65,921,112 stars). The colour scale rep- age and chemical composition. resents the square root of the relative density of stars.
3.1. Membership and astrometric solutions archive query combining all the filters presented here is provided Two types of astrometric solutions were applied. The first type is in Appendix B. applicable to nearby clusters. For the second Gaia data release, the nearby ‘limit’ was set at 250 pc. Within this limit, the par- allax and proper motion data for the individual cluster members 2.2. Extinction are sufficiently accurate to reflect the effects of projection along the line of sight, thus enabling the 3D reconstruction of the clus- The dust that is present along the line of sight towards the stars ter. This is further described in Appendix A.1. leads to a dimming and reddening of their observed light. In the For these nearby clusters, the size of the cluster relative to full colour - absolute magnitude diagram presented in Fig. 1, the its distance will contribute a significant level of scatter to the effect of the extinction is particularly striking for the red clump. HRD if parallaxes for individual cluster members are not taken The de-reddened HRD using the extinction provided together into account. With a relative accuracy of about 1% in the par- with DR2 is presented in Andrae et al. (2018). To study the fine allax measurement, an error contribution of around 0.02 in the structures of the Gaia HRD for field stars, we selected here only absolute magnitude is possible. For a large portion of the Gaia low-extinction stars. High galactic latitude and close-by stars lo- photometry, the uncertainties are about 5 to 10 times lower, mak- cated within the local bubble (the reddening is almost negligible ing the parallax measurement still the main contributor to the within ∼60 pc of the Sun (Lallement et al. 2003)) are affected uncertainty in the absolute magnitude. The range of differences less from the extinction, and we did not apply further selection in parallax between the cluster centre and an individual cluster for them. To select low-extinction stars away from these simple member depends on the ratio of the cluster radius over the cluster cases, we followed Ruiz-Dern et al. (2018) and used the 3D ex- distance. At a radius of 15 pc, the 1% level is found for a cluster 1 tinction map of Capitanio et al. (2017) , which is particularly at 1.5 kpc, or a parallax of 0.67 mas. In Gaia DR2, formal un- well adapted to finding holes in the interstellar medium and to certainties on the parallaxes may reach levels of just lower than select field stars with E(B − V) < 0.015. 10 µas, but the overall uncertainty from localised systematics is For globular clusters we used literature extinction values about 0.025 mas. If this value is considered the 1% uncertainty (Sect. 3.3), while for open clusters, they are derived together level, then a resolution of a cluster along the line of sight, us- with the ages (Sect. 3.2). Detailed comparisons of these global ing Gaia DR2, becomes possible for clusters within 400 pc, and cluster extinctions with those that can be derived from the ex- realistic for clusters within about 250 pc. tinctions provided by Gaia DR2 can be found in Arenou et al. For clusters at larger distances, the mean cluster proper mo- (2018). To transform the global cluster extinction easily into the tion and parallax are derived directly from the observed astro- Gaia passbands while taking into account the extinction coeffi- metric parameters for the individual cluster members. The de- cients dependency on colour and extinction itself in these large tails of this procedure are presented in Appendix A.2. passbands (e.g. Jordi et al. 2010), we used the same formulae as Danielski et al. (2018) to compute the extinction coefficients 3.2. Selection of open clusters Our sample of open clusters consists of the mostly well-defined 1 http://stilism.obspm.fr/ and fairly rich clusters within 250 pc, and a selection of mainly
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Fig. 2. Composite HRD for 32 open clusters, coloured according to log(age), using the extinction and distance moduli as determined from the Gaia data (Table 2).
Fig. 3. Composite HRD for 14 globular clusters, coloured according to metallicity (Table 3).
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Table 1. Parameters used to derive the Gaia extinction coefficients as a function of colour and extinction (Eq. 1).
c1 c2 c3 c4 c5 c6 c7 kG 0.9761 -0.1704 0.0086 0.0011 -0.0438 0.0013 0.0099 kBP 1.1517 -0.0871 -0.0333 0.0173 -0.0230 0.0006 0.0043 kRP 0.6104 -0.0170 -0.0026 -0.0017 -0.0078 0.00005 0.0006 rich clusters at larger distances, covering a wider range of ages, Table 2. Overview of reference values used in constructing the compos- mostly up to 1.5 kpc, with a few additional clusters at larger dis- ite HRD for open clusters (Figure 2). tances where these might supply additional information at more extreme ages. For very young clusters, the definition of the clus- Cluster DM log(age) [Fe/H] E(B−V) Memb ter is not always clear, as the youngest systems are mostly found Hyades 3.389 8.90 0.13 0.001 480 embedded in OB associations, producing large samples of sim- Coma Ber 4.669 8.81 0.00 0.000 127 ilar proper motions and parallaxes. Very few clusters appear to Pleiades 5.667 8.04 -0.01 0.045 1059 survive to an ‘old age’, but those that do are generally rich, al- IC 2391 5.908 7.70 -0.01 0.030 254 lowing good membership determination. The final selection con- IC 2602 5.914 7.60 -0.02 0.031 391 sists of 9 clusters within 250 pc, and 37 clusters up to 5.3 kpc. Of α Per 6.214 7.85 0.14 0.090 598 the latter group, only 23 were finally used for construction of the Praesepe 6.350 8.85 0.16 0.027 771 colour-magnitude diagram; these clusters are listed together with NGC 2451A 6.433 7.78 -0.08 0.000 311 the 9 nearby clusters in Table 2. For the remaining 14 clusters, Blanco 1 6.876 8.06 0.03 0.010 361 the colour-magnitude diagrams appeared to be too much affected NGC 6475 7.234 8.54 0.02 0.049 874 by interstellar reddening variations. More details on the astro- NGC 7092 7.390 8.54 0.00 0.010 248 metric solutions are provided in Appendix A; the solutions are NGC 6774 7.455 9.30 0.16 0.080 165 presented for the nearby clusters in Table A.3 and for the more NGC 2232 7.575 7.70 0.11 0.031 241 distant clusters in Table A.4. Figure 2 shows the combined HRD NGC 2547 7.980 7.60 -0.14 0.040 404 of these clusters, coloured according to their ages as provided NGC 2516 8.091 8.48 0.05 0.071 1727 in Table 2. The main-sequence turn-off and red clump evolution Trumpler 10 8.223 7.78 -0.12 0.056 407 with age is clearly visible. The age difference is also shown for NGC 752 8.264 9.15 -0.03 0.040 259 lower mass stars, the youngest stars lie slightly above the main NGC 6405 8.320 7.90 0.07 0.139 544 sequence of the others. The white dwarf sequence is also visible. IC 4756 8.401 8.98 0.02 0.128 515 NGC 3532 8.430 8.60 0.00 0.022 1802 NGC 2422 8.436 8.11 0.09 0.090 572 3.3. Selection of globular clusters NGC 1039 8.552 8.40 0.02 0.077 497 NGC 6281 8.638 8.48 0.06 0.130 534 The details of selecting globular clusters are presented in NGC 6793 8.894 8.78 0.272 271 Gaia Collaboration et al. (2018c). A major issue for the globular NGC 2548 9.451 8.74 0.08 0.020 374 cluster data is the uncertainties on the parallaxes that result from NGC 6025 9.513 8.18 0.170 431 the systematics, which is in most cases about one order of mag- NGC 2682 9.726 9.54 0.03 0.037 1194 nitude larger than the standard uncertainties on the mean paral- IC 4651 9.889 9.30 0.12 0.040 932 lax determinations for the globular clusters. The implication of NGC 2323 10.010 8.30 0.105 372 this is that the parallaxes as determined with the Gaia data can- NGC 2447 10.088 8.74 -0.05 0.034 681 not be used to derive the distance moduli needed to prepare the NGC 2360 10.229 8.98 -0.03 0.090 848 composite HRD for the globular clusters. Instead, we had to rely NGC 188 11.490 9.74 0.11 0.085 956 on distances as quoted in the literature, for which we used the tables (2010 edition) provided online by Harris (1996). The in- Notes. Distance moduli (DM) as derived from the Gaia astrome- Gaia evitable drawback is that these distances and reddening values try; ages and reddening values as derived from photometry (see Sect. 6), with distances fixed on astrometric determinations; metallic- have been obtained through isochrone fitting, and the applica- ities from Netopil et al. (2016); Memb: the number of members with tion of these values to the Gaia data will provide only limited Gaia photometric data after application of the photometric filters. new information. The main advantage is the possibility of com- paring the HRDs of all globular clusters within a single, accurate photometric system. The combined HRD for 14 globular clusters clusters are much younger, while most globular clusters are is shown in Fig. 3, the summary data for these clusters is pre- much less metal rich. sented in Table 3. The photometric data originate predominantly from the outskirts of the clusters, as in the cluster centres the crowding often affects the colour index determination. Figure 3 shows the blue horizontal branch populated with the metal-poor 4. Details of the Gaia HRDs clusters and the move of the giant branch towards the blue with In the following, several field star HRDs are presented. Unless decreasing metallicity. otherwise stated, the filters presented in Sect. 2.1, including the An interesting comparison can be made between the E(B−V) < 0.015 mag criteria, were applied. The HRDs use a red most metal-rich well-populated globular cluster of our sample, colour scale that represents the square root of the density of stars. 47 Tuc (NGC 104), and one of the oldest open clusters, M67 The Gaia DR2 HRD of the low-extinction stars is represented (NGC 2682) (Fig. 4). This provides the closest comparison be- in Fig. 5. The approximate equivalent temperature and luminos- tween the HRDs of an open and a globular cluster. Most open ity to the GBP − GRP colour and the absolute Gaia MG magni-
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Table 3. Reference data for 14 globular clusters used in the construction of the combined HRD (Figure 3).
NGC DM Age [Fe/H] E(B−V) Memb (Gyr) 104 13.266 12.751 -0.72 0.04 21580 288 14.747 12.501 -1.31 0.03 1953 362 14.672 11.501 -1.26 0.05 1737 1851 15.414 13.303 -1.18 0.02 744 5272 15.043 12.602 -1.50 0.01 9086 5904 14.375 12.251 -1.29 0.03 3476 6205 14.256 13.001 -1.53 0.02 10311 6218 13.406 13.251 -1.37 0.19 3127 6341 14.595 13.251 -2.31 0.02 1432 6397 11.920 13.501 -2.02 0.18 10055 6656 12.526 12.863 -1.70 0.35 9542 6752 13.010 12.501 -1.54 0.04 10779 6809 13.662 13.501 -1.94 0.08 8073 7099 14.542 13.251 -2.27 0.03 1016
Notes. Data on distance moduli (DM), [Fe/H] and E(B−V) from Har- ris (1996), 2010 edition, (1): Dotter et al. (2010), (2) Denissenkov et al. (2017), (3) Powalka et al. (2017) for age estimates. Memb: cluster mem- bers with photometry after application of photometric filters.
Fig. 5. Gaia HRD of sources with low extinction (E(B − V) < 0.015 mag) satisfying the filters described in Sect. 2.1 (4,276,690 stars). The colour scale represents the square root of the density of stars. Ap- proximate temperature and luminosity equivalents for main-sequence stars are provided at the top and right axis, respectively, to guide the eye.
tainties in the parallax of the individual stars, and the underlying main sequence is likely to be even narrower. The binary sequence spread is visible throughout the main Fig. 4. Comparison between the HRDs of 47 Tuc (NGC 104, sequence (Figs. 5 and 6), and most clearly in open clusters Age=12.75 Gyr, [Fe/H]=-0.72), one of the most metal-rich globu- (Fig. 7, see also Sect. 6). It is most preeminent for field stars be- lar clusters (magenta dots), and M 67 (NGC 2682, Age=3.47 Gyr, low MG = 13. Figure 8 shows the main-sequence fiducial of the [Fe/H]=0.03), one of the oldest open clusters (blue dots). local HRD shifted by 0.753 mag, which corresponds to two iden- tical stars in an unresolved binary system observed with the same colour but twice the luminosity of the equivalent single star. See tude provided in the figure were determined using the PARSEC Hurley & Tout (1998), for instance, for a discussion of this strong isochrones (Marigo et al. 2017) for main-sequence stars. sequence. Binaries with a main-sequence primary and a giant Figure 6 shows the local Gaia HRDs using several cuts in companion would lie much higher in the diagram, while binaries parallax, still with the filters of Sect. 2.1, but without the need with a late-type main-sequence primary and a white dwarf com- to apply the E(B − V) < 0.015 mag extinction criteria, as these panion lie between the white dwarf and the main sequence, as is sources mostly lie within the local bubble. shown in Fig. 5, for example. The main sequence is thicker between 10 < MG < 13 (Figs. 2, 5, 6). The youngest main-sequence stars lie on the up- 4.1. Main sequence per part of the main sequence (in blue in Fig. 2). The subdwarfs, The main sequence is very thin, both in fields and in clusters. which are metal-poor stars associated with the halo, are visible This is very clearly visible in Fig. 7, which shows the HRDs of in the lower part of the local HRD (in red in Fig. 2, see also the Hyades and Praesepe clusters (ages ∼700 Myr), which accu- Sect. 7). rately overlap, as has previously been noticed in van Leeuwen The main-sequence turn-off variation with age is clearly il- (2009) and confirmed in Gaia Collaboration et al. (2017). This lustrated in Fig. 2, and the variation with metallicity is shown in figure shows the very narrow sequence described by the stars in Fig. 3. Blue stragglers are also visible over the main-sequence both clusters, as well as the scattering of double stars up to 0.75 turn-off (Figs. 4). magnitudes above the main sequence. The remaining width of Between the main sequence and the subgiants lies a tail of the main sequence is still largely explained as due to the uncer- stars around MG = 4 and GBP − GRP=1.5. These stars shows
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Fig. 6. Solar neighbourhood Gaia HRDs for a) $ > 40 mas (25 pc, 3,724 stars), b) $ > 20 mas (50 pc, 29,683 stars), and c) $ > 10 mas (100 pc, 212,728 stars).
Fig. 7. Extract of the HRD for the Hyades and Praesepe clusters, show- ing the detailed agreement between the main sequences of the two clus- ters, the narrowness of the combined main sequence, and a scattering of double stars up to 0.75 mag above the main sequence. variability and may be associated with RS Canum Venaticorum variables, which are close binary stars (Gaia Collaboration et al. 2018b).
4.2. Brown dwarfs Fig. 8. Same as Fig. 6c, overlaid in blue with the median fiducial and in green with the same fiducial shifted by -0.753 mag, corresponding to an To study the location of the low-mass objects in the Gaia HRD, unresolved binary system of two identical stars. we used the Gaia ultracool dwarf sample (GUCDS) compiled by Smart et al. (2017). It includes 1886 brown dwarfs (BD) of L, T, and Y types, although a substantial fraction of them are too faint for Gaia. We note that the authors found 328 BDs in common with the Gaia DR1 catalogue (Gaia Collaboration et al. 2016). and relative parallax errors smaller than 25%. They also have as- The crossmatch between the 2MASS catalogue (Skrutskie trometric excess noise larger than 1 mas and a high (IBP +IRP)/IG et al. 2006) and Gaia DR2 provided within the Gaia archive flux ratio. They are faint red objects with very low flux in the (Marrese et al. 2018) has been used to identify GUCDS entries. BP wavelength range of their spectrum. Any background under- The resulting sample includes 601 BDs. Of these, 527 have five- estimation causes the measured BP flux to increase to more than parameter solutions (coordinates, proper motions, and parallax) it should be, yielding high flux ratios, the highest ratios are de- and full photometry (G, GBP, and GRP). Most of these BDs have rived for the faintest BDs. The filters presented in Sect. 2.1 there- parallaxes higher than 4 mas (equivalent to 250 pc in distance) fore did not allow us to retain them.
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Fig. 9. a): Gaia HRD of the stars with $ > 10 mas with adapted photometric filters (see text, 240,703 stars) overlaid with all cross-matched GUCDS (Smart et al. 2017) stars with σ$/$ < 10% in blue (M type), green (L type), and red (T type). Pink squares are added around stars with −1 tangential velocity VT>200 km s . b) BT-Settl tracks (Baraffe et al. 2015) of solar metallicity for masses from 0.01 M to 0.08 M in steps of 0.01 (the upper tracks correspond to lower masses) plus in pink the same tracks for [M/H]= -1.0. Panels c) and d): Same diagrams using the 2MASS colours.
We accordingly adapted our filters for the background stars masses < 0.08 M that were computed using the nominal Gaia of Fig. 9. We plot the HRD using the G − GRP colour instead passbands. With the Gaia G − GRP colour, the sequence of M, L, of GBP − GRP because of the poor quality of GBP for these faint T types is continuous and relatively thin. Conversely, the spread red sources. We applied the same astrometric filters as for Fig. in the near-infrared is larger and the L/T transition feature is 6c, but we did not filter the fluxes ratio or the GBP photometric strongly seen with a shift of J − Ks to the blue, which is due uncertainties. More dispersion is present in this diagram than in to a drastic change in the brown dwarf cloud properties (e.g. Fig. 6c because of this missing filter, but the faint red sources we Saumon & Marley 2008). Some GUCDS L-type stars with very study here are represented better. blue 2MASS colours seem at first sight intriguing, but their lo- The 470 BDs for which DR2 provides parallaxes better than cation might be consistent with metal-poor tracks (Fig. 9d). Fol- 10%, and the G and GRP magnitudes are overlaid in Fig. 9 with- lowing Faherty et al. (2009), we studied their kinematics, which out any filtering. The sequence of BDs follows the sequence are indeed consistent with the halo kinematic cut of the tangen- −1 of low-mass stars. The absolute magnitudes of four stars are tial velocity VT>200 km s (see Sect. 7). A kinematic selection too bright, most probably because of a cross-match issue. In of the global HRD as done in Sect. 7 but using the 2MASS Fig. 9a the M-, L- and T-type BDs are sorted according to colours confirms the blue tail of the bottom of the main sequence the classification in GUCDS. There are 21, 443, and 7 of each in the near-infrared for the halo kinematic selection. type, respectively. We also present in Fig. 9c the correspond- ing HRD using 2MASS colours with the 2MASS photometric 4.3. Giant branch quality flag AAA (applied to background and GUCDS stars). Figures 9b,d includes BT-Settl tracks2 (Baraffe et al. 2015) for The clusters clearly illustrate the change in global shape of the giant branch with age and metallicity (Figs. 2 and 3). For field 2 https://phoenix.ens-lyon.fr/Grids/BT-Settl/CIFIST2011bc stars, there are fewer giants than dwarfs in the first 100 pc. To
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populations. Mucciarelli et al. (2016) concluded that most of the stars that populate the red HB are Na poor and belong to the first generation, while the blue side of the HB is populated by the Na-rich stars belonging to the second generation. The same kind of correlation is shown in general by the globular clusters. We quote among others the studies of 47 Tuc (Gratton et al. 2013) and NGC 6397 (Carretta et al. 2009). The role of the He abun- dances is still under discussion (Valcarce et al. 2016; Marino et al. 2014). He-enhanced stars are indeed expected to populate the blue side of the instability strip because they are still O de- pleted and Na enhanced, as observed in the second-generation stars. How significant the He enhancement is is still unclear. Figure 3 shows that the globular cluster HB can extend to- wards the extreme horizontal branch (EHB) region. They are in the same region of the HRD as the hot subdwarfs, which creates a clump at MG=4 and GBP−GRP=-0.5 that is well visible in Fig. 1 and Fig. 5. These stars are also nicely characterised in terms of variability, including binary-induced variability, in Gaia Collab- Fig. 10. Gaia HRD of low-extinction nearby giants: $ > 2 mas oration et al. (2018b). These hot subdwarfs are considered to be red giants that lost their outer hydrogen layers before the core be- (500 pc), E(B − V) < 0.015 and MG < 2.5 (29288 stars), with labels to the features discussed in the text. gan to fuse helium, which might be due to the interaction with a low-mass companion, although other processes might be at play (e.g. Heber 2009). Gaia will allow detailed studies of the differ- observe the field giant branch in more detail, we therefore ex- ences between cluster and field hot subdwarfs. tended our selection to 500 pc with the low-extinction selection (E(B − V) < 0.015, see Sect. 2.2) for Fig. 10. The most prominent feature of the giant branch is the Red 4.4. Planetary nebulae Clump (RC in Fig. 10, around GBP − GRP= 1.2, MG = 0.5 mag). It corresponds to low-mass stars that burn helium in their core At the end of the AGB phase, the star has lost most of its hydro- (e.g. Girardi 2016). The colour of core-helium burning stars is gen envelope. The gas expands while the central star first grows strongly dependent on metallicity and age. The more metal-rich, hotter at constant luminosity, contracting and fusing hydrogen in the redder, which leads to this red clump feature in the local the shell around its core (post-AGB phase), then it slowly cools HRD. For more metal-poor populations, these stars are bluer and when the hydrogen shell is exhausted, to reach the white dwarf lead to the horizontal-branch (HB) feature that is clearly visible phase. This planetary nebulae phase is very short, about 10,000 in globular clusters (Fig. 11). years, and is therefore quite difficult to observe in the HRD. The The secondary red clump (SRC in Fig. 10, around GBP − Gaia DR2 contains many observations of nearby planetary neb- GRP= 1.1, MG = 0.6) is more extended in its bluest part to fainter ulae as their expanding gas create excess flux over the mean sky magnitudes than the red clump. It corresponds to younger more background that triggers the on-board detection. We here wish to massive red clump stars (Girardi 1999) and is therefore mostly follow the route of the central star in the HRD. While some cen- visible in the local HRD (Fig. 6c). Core-helium burning stars that tral planetary nebula stars are visible in the Galactic Pole HRD are even more massive are more luminous than the red clump and (Fig. 12), post-AGB stars are too rare to appear in this diagram. lie still on the blue part of it, leading to a vertical structure that We used catalogue compilations to highlight the position of the is sometimes called the Vertical Red Clump (VRC in Fig. 10). two types in the Gaia HRD. On the red side and fainter than the clump lies the RGB bump (RGBB in Fig. 10). This bump is caused by a brief interruption We used the Kerber et al. (2003) catalogue of Galactic plan- of the stellar luminosity increase as a star evolves on the red giant etary nebulae, selecting only sources classified as central stars branch by burning its hydrogen shell, which creates an accumu- that are clearly separated from the nebula. With a cross-match 00 lation of stars at this HRD position (e.g. Christensen-Dalsgaard radius of 1 and using all our filter criteria of Sect. 2.1, only 2015). Its luminosity changes more with metallicity and age than four stars remain. We therefore relaxed the extinction criteria − the red clump. Brighter than the red clump, at MG ∼ −0.5, to E(B V) < 0.05 and the parallax relative uncertainty to lies the AGB bump (AGBB in Fig. 10), which corresponds to σ$/$ < 20%, leading to 23 stars. the start of the asymptotic giant branch (AGB) where stars are For post-AGB stars, we used the catalogue of Szczerba et al. burning their helium shell (e.g. Gallart 1998). The AGB bump (2007) and the 2MASS identifier provided for the cross-match. is much less densely populated than the RGB bump. It is also We selected only stars that are classified as very likely post-AGB clearly visible in the HRD of 47 Tuc (Fig. 11a). objects. Here we also relaxed the extinction criteria to E(B − The globular clusters in Fig. 11 clearly illustrate the di- V) < 0.05 and the parallax relative uncertainty to σ$/$ < 20%, versity of the HB morphology. Some have predominantly blue leading to 11 stars. HB (NGC 6397), some just red HB (NGC 104), and some a mixed HB showing bimodal distribution (NGC 5272 and While some outliers are seen in Fig. 12, either due to cross- NGC 6362). The HB morphology is explained in the framework match or misclassification issues, the global position of these of the multiple populations; it is regulated by age, metallicity, stars in the HRD closely follows the expected track from the and first/second generation abundances (Carretta et al. 2009). AGB to the white dwarf sequence. We note that this path crosses NGC 6362 is the least massive globular that presents multiple the hot subdwarf region we discussed in the previous section.
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Fig. 11. Several globular clusters selected to show a clearly defined and very different horizontal branch, sorted by decreasing metallicity. a) NGC 104 (47 Tuc), b) NGC 6362, c) NGC 5272, and d) NGC 6397.
Fig. 13. Gaia HRD of white dwarfs with σ$/$ < 5% (26,264 stars), with letter labels to the features discussed in the text.
more stars, but the uncertainties remain too large to constrain the Fig. 12. North Galactic Pole HRD (b > 50◦, 2,077,925 stars) with litera- theoretical mass-radius relations. Only in a few cases, where the ture central planetary nebula stars (blue) and post-AGB stars (magenta). white dwarf resides in a binary system, have mass radius mea- surements begun to approach the accuracy required to constrain the core composition and H layer mass of individual stars (e.g. 5. White dwarfs Barstow et al. 2005; Parsons et al. 2017; Joyce et al. 2017). Even The Sloan Digital Sky Survey (SDSS, Ahn et al. 2012) has pro- then, some of these white dwarfs may not be representative of duced the largest spectroscopic catalogue of white dwarfs so far the general population because common envelope evolution may (e.g. Kleinman et al. 2013). This data set has greatly aided our have caused them to depart from the normal white dwarf evolu- understanding of white dwarf classification and evolution. For tionary paths. example, it has allowed determining the white dwarf mass dis- The publication of Gaia DR2 presents the opportunity to ap- tribution for large statistical samples of different white dwarf ply accurate parallaxes, with uncertainties of 1% or smaller, to spectral types. However, much of this work is model dependent the study of white dwarf stars. The availability of these data, and relies upon theoretical mass-radius relationships and stel- coupled with the accurate Gaia photometry, yields the absolute lar atmosphere models, whose precision has only been tested in magnitude, with which the white dwarfs can be clearly located in a limited way. These tests have been limited by the relatively the expected region of the HRD (Figs. 5, 6). Figure 13 shows the small number of white dwarfs for which accurate parallaxes are white dwarf region of the HRD alone. This sample was selected available (e.g. Provencal et al. 1998) and by the precision of with GBP −GRP < 2 and G−10+5 log10 $ > 10+2.6 (GBP −GRP) the parallaxes for these faint stars. This work was updated using and by applying the filters described in Sect. 2, including the the Gaia DR1 catalogue (Tremblay et al. 2017), which included low-extinction E(B − V) < 0.015 criterion, but with a stronger
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Fig. 15. SDSS white dwarfs (5,237 stars) with evolutionary models. Mu is computed using the SDSS u magnitude and the Gaia parallax. Magenta: 0.6 M pure H; green dashed: 0.8 M pure H; and blue : 0.6 M pure He.
Just below the main 0.6 M concentration of white dwarfs is a second, separate concentration (B) that seems to be sepa- rate from the 0.6 M peak at GBP − GRP ∼ −0.1 before again merging by GBP − GRP ∼ 0.8. At the maximum separation, this concentration is roughly aligned with the 0.8 M hydrogen white dwarf cooling track (in green), which is not expected. While the SDSS mass distribution (Kleinman et al. 2013) shows a signifi- cant upper tail that extends through 0.8 M and up to almost 1.2 M , there is no evidence for a minimum between 0.6 and 0.8 M like that seen in Fig. 14a. A mass difference should therefore not lead to this feature. However, for a given mass, the evolution- ary tracks for different compositions (DA: hydrogen and DB: helium) and envelope masses are virtually coincident at the res- olution of Fig. 14 in the theoretical tracks, leading to no direct interpretation from the tracks in the HRD alone, but we describe Fig. 14. Gaia HRD of white dwarfs with σ /$ < 5% and σ < 0.01 $ GBP below a different view from the colour-colour relation and the and σ < 0.01 (5,781 stars) overlaid with white dwarf evolutionary GRP SDSS comparison. models. Magenta: 0.6 M pure H; green dashed: 0.8 M pure H; and blue: 0.6 M pure He. a): HRD. b): Colour-colour diagram. A third, weaker concentration of white dwarfs in Fig. 13 lies below the main groups (Q). It does not follow an obvious evo- lutionary constant mass curve, which would be parallel to those constraint on the parallax relative uncertainty of 5%. This yields shown in the plot. Beginning at approximately MG = 13 and a catalogue of 26,264 objects. We overplot in Fig. 14 white dwarf GBP − GRP = −0.3, it follows a shallower curve that converges evolutionary models3 for C/O cores (Holberg & Bergeron 2006; with the other concentrations near GBP − GRP = 0.2. Kowalski & Saumon 2006; Tremblay et al. 2011; Bergeron et al. White dwarfs are also seen to lie above the main concentra- 2011) with colours computed using the revised Gaia DR2 pass- tion A. This can be explained as a mix between natural white bands (Evans et al. 2018). dwarf mass distributions and binarity (see Fig. 8). Several features are clearly visible in Fig. 13. First there is a Selecting only the most precise GBP and GRP photometry clear main concentration of stars that is distributed continuously (σGBP < 0.01 and σGRP < 0.01), we examined the colour-colour from left to right in the diagram (A) and coincides with the 0.6 relation in Fig. 14b. The sequence is also split into two parts in M hydrogen evolutionary tracks (in magenta). This is expected this diagram. We verified that the two splits coincide, meaning because the white dwarf mass distribution peaks very strongly that the stars in the lower part of Fig. 14a lie in the upper part near 0.6 M (Kleinman et al. 2013). Interestingly, the concentra- of Fig. 14b. The mass is not expected to lead to significant dif- tion of white dwarfs departs from the cooling tracks towards the ferences in this colour-colour diagram, and the theoretical tracks red end of the sequence. coincide with the observed splits, pointing towards a difference between helium and hydrogen white dwarfs. It also recalls the 3 http://www.astro.umontreal.ca/~bergeron/CoolingModels split in the SDSS colour-colour diagram (Harris et al. 2003).
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While Gaia identifies white dwarfs based on their location on the HRD, SDSS white dwarfs were identified spectroscopically, providing further information on the spectral type, Teff, and log g as well as a classification. Therefore we cross-matched the two data sets to better understand the features observed in Fig. 14. We obtained a catalogue of spectroscopically identified SDSS white dwarfs from the Montreal White Dwarf Database4 (Dufour et al. 2017) by downloading the whole catalogue and then filtering for SDSS identifier, which yielded 28,797 objects. Using the SDSS cross-match provided in the Gaia archive (Mar- rese et al. 2018), we found that there are 22,802 objects in com- mon and 5,237 satisfying all the filters described in Sect. 2.1 and with single-star spectral type information. Figure 15 shows the SDSS u − g colour magnitude for the sample with the absolute u magnitude calculated using the Gaia parallax. The distribution is clearly bifurcated. Evolutionary tracks for H and He atmo- spheres (0.6 M ) are overplotted in the figure, indicating that this is due to the different atmospheric compositions. The Gaia coun- terparts of these SDSS white dwarfs are quite faint, and therefore the features seen in Fig. 14a are less well visible in this sample because of the larger noise in the parallaxes and the colours. Still, it allowed us to verify that the split of the SDSS white dwarfs corresponds to the location of the Gaia splits in Fig. 14. The narrower filter bands of SDSS are more sensitive to atmospheric compositions than the broad BP and RP Gaia bands. In particu- lar, the u -band fluxes of H-rich DA white dwarfs are suppressed by the Balmer jump at 364.6 nm, which reddens the colours of these stars. The Balmer jump is in the wavelength range where the Gaia filters calibrated for DR2 differ most from the nominal filters (Evans et al. 2018), which explains the importance of us- ing tracks that are updated to the DR2 filters for the white dwarf studies instead of the nominal tracks provided by Carrasco et al. (2014). Figure 16 shows the colour-magnitude diagrams in the Gaia and SDSS photometry bands, overlaid with the white dwarfs for Fig. 16. SDSS white dwarfs per spectral type (DA: hydrogen; DB: neu- specific spectral types. The locations of the various spectral types tral helium; DO: ionised helium; DQ: carbon; DZ: metal rich; and DC: correspond well to the expected colours arising from their effec- no strong lines). panel a): Gaia photometry, panel b): SDSS photometry. tive temperatures. For example, DQ (carbon), DZ (metal rich), and DC (no strong lines) stars are confined to the red end of the colour-magnitude diagram, while the DO stars (ionised helium) tal properties for a meaningful description of the disk charac- all lie at the blue end. DAs cover the whole diagram. Interest- teristics. Their study complements the field population studies ingly, in Fig. 16b, a significant number of classified DA white that are based on Galactic surveys. Globular clusters are funda- dwarfs appears to occupy the He-rich atmosphere branch that is mental tools for studying the properties of low-mass stars and indicated by the evolutionary track in Fig. 15. The weaker Q con- the early chemical evolution of the Galaxy. Now Gaia DR2 data centration seems to include stars of all types except for DO and bring us into a completely new domain. High-accuracy paral- DZ. However, the most numerous components are the DA and laxes and exquisite photometry make the comparison with the- DQs. oretical isochrones very fruitful, based on which, stellar proper- ties can be defined. A detailed discussion of the uncertainties of stellar models is beyond the scope of this paper. Here we 6. Cluster as stellar parameter templates would like to recall that effects such as convection in the stel- Clusters have long been considered as benchmarks with regard lar core, mass loss, rotation, and magnetic fields are still poorly to the determination of the stellar properties. Open cluster stars constrained and are often only parametrised in stellar models share common properties, such as age and chemical abundances. (Bell 2016; Pasetto et al. 2016; Weiss & Heners 2013). Al- The level of homogeneity of open clusters has been assessed though very significant, seismic predictions depend on our poor in several papers (Cantat-Gaudin et al. 2014; Bovy 2016). By knowledge of the relevant physics (Miglio et al. 2015). A cal- means of a high-precision differential abundance analysis, the ibration of these effects on star cluster photometry is manda- Hyades have been proved to be chemically in-homogeneous at tory and will complement asteroseismology as a tool for test- the 0.02 dex level (Liu et al. 2016) at maximum. Until now, the ing stellar physics and will ultimately improve stellar models. study of clusters was hampered by the disk field contamination. In Table 2 we present the ages and the extinction values derived This in turn results in difficult membership determination, and by isochrone fitting for the sample of open clusters discussed in +0.14 +0.11 +0.08 in highly uncertain parameters (Netopil et al. 2015). Distance this paper. The uncertainties are ∆(log(age)) −0.22, −0.13, −0.06 for and age, together with chemical abundances, are the fundamen- 6 < log(age) ≤ 7, 7 < log(age) ≤ 8, log(age) > 8, respec- tively, and ∆E(B − R) = 0.04. Here we made use of PARSEC 4 http://www.montrealwhitedwarfdatabase.org/ isochrones (Chen et al. 2014) for metallicities Z = 0.017 and
Article number, page 13 of 28 A&A proofs: manuscript no. DR2HRD
Fig. 17. HRDs of nearby clusters compared with PARSEC isochrones (see text for details) of the Pleiades (a), Praesepe (b), Coma Ber (c), Hyades (d), Alpha Per (e), and Blanco 1 (f). Praesepe, Hyades, and Alpha Per are fitted with Z = 0.02, while the others are reproduced using Z = 0.017.
Z = 0.020 updated to the latest transmission curve calibrated on oration et al. 2017; Kharchenko et al. 2015), while the others Gaia DR2 data (Evans et al. 2018)5. Praesepe, Hyades, Alpha were reproduced using Z = 0.017. This gave a relatively poor fit Per, and NGC 6475 were fitted with Z = 0.02 (Gaia Collab- for clusters that are known to have sub-solar metallicity, such as NGC 2158, which has [Fe/H]=-0.25 (Kharchenko et al. 2015). 5 PARSEC isochrones in Gaia DR2 passbands are available at http: The PARSEC solar value is Z = 0.015. This version of the PAR- //stev.oapd.inaf.it/cgi-bin/cmd SEC tracks makes use of a modified relation between the effec-
Article number, page 14 of 28 Gaia Collaboration et al.: Gaia Data Release 2: Observational Hertzsprung-Russell diagrams
Fig. 18. HRDs of two distant clusters compared with PARSEC isochrones (see text for details): M67 (NGC 2682) (a), and NGC 2447(b).
to be slightly steeper than the observed main sequence, while at even lower magnitudes, the slope of the observed main sequence becomes steeper than the predicted main sequence. The latter ef- fect might be due to the background subtraction, which becomes challenging at these faint magnitudes (Evans et al. 2018; Are- nou et al. 2018). Instead, the initial steepening of the isochrones, which is also observed in other clusters in Fig. 17, might indi- cate that the adopted boundary conditions in the domain of very low mass stars in PARSEC need a further small revision. It is well known that current models and the colour transformations fail to reproduce the main sequence in the very low mass regime (Bell 2016), and the data gathered by Gaia will certainly help to overcome this long-standing problem. The age determination of Blanco 1 deserves further com- ments. Blanco 1 has a slightly super-solar metallicity [Fe/H] = +0.04 ± 0.04 (Ford et al. 2005). Previous age determination placed Blanco 1 in the age range log(age) = 8.0 − 8.17 (Moraux et al. 2007). A determination of the lithium depletion boundary Fig. 19. HRD of the globular cluster 47 Tuc compared with PARSEC on very low mass stars gives log(age) = 8.06 ± 0.13 when a cor- isochrones (see text for details). The inner region (radius < 10 arcmin, e.g. three times the half-light radius) is shown in green, while the exter- rection for magnetic activity is applied (Juarez et al. 2014). From nal regions are plotted in blue. A maximum radius of 1.1 degrees was the main-sequence turnoff, we obtain log(age) = 8.30. However, used. fitting the main-sequence turnoff in such an inconspicuous clus- ter might not lead to correct results, since the initial mass func- tion disfavours higher mass stars. Using the lithium depletion tive temperature and Rosseland mean optical depth τ across the boundary age of log(age) of 8.06 ± 0.04, we reproduce the lower atmosphere that is derived from PHOENIX (Allard et al. 2012) main sequence, with a marginal fit to the upper main sequence. and in particular from the set of BT-Settl models. With this mod- Similar considerations apply to the Pleiades, whose log(age) is ified relation, introduced to better reproduce the observed mass- in the range 8.04 ± 0.03 - 8.10 ± 0.06 and is derived from the radius relation in nearby low mass stars (Chen et al. 2014), the lithium depletion boundary or from eclipsing binaries (for a re- models provide a good representation of the colour distribution cent discussion, see David et al. 2016). Using the lithium deple- of very low mass stars in several passbands. tion boundary age of 8.04 ± 0.06, we can reproduce the main Figure 17 shows the HRD of a few nearby open clusters com- sequence with PARSEC isochrones. pared with PARSEC isochrones. The distance modulus (Table 2) Figure 18 presents the comparison of two distant clusters, was used, and the extinction was not corrected in the photometry, NGC 2682 (M67) and NGC 2447, with PARSEC isochrones. but was applied on the isochrones. M 67 is one of the best-studied star clusters. It has a metallic- The fits are remarkably good in the upper and lower main ity near solar, an accessible distance of about 1028 pc with low sequence. The high quality of Gaia photometry produces well- reddening (Taylor 2007), and an age close to solar (∼ 4 Gyr). It defined features, very clean main sequences, and a clear defini- is a very highly populated object that includes over 1000 mem- tion of the binary sequence. The agreement for the Pleiades is bers from main-sequence dwarfs, a well-populated subgiant and particularly remarkable. In spite of the impressively good agree- red giant branch, white dwarfs, blue stragglers, sub-subgiants, ment across a range of several magnitudes, at about MG ∼ 10, X-ray sources, and cataclysmic variables. Gaia identifies 1526 the model predictions and the observed main sequence still dis- members. It was observed by almost all the most relevant spec- agree. The slope of the theoretical main sequence initially seems troscopic surveys (Gaia-ESO, APOGEE, WIYN, etc.). Astero-
Article number, page 15 of 28 A&A proofs: manuscript no. DR2HRD seismologic data are available from the Kepler 2 mission (Stello ties are not included in Gaia DR2 (Sartoretti et al. 2018), which et al. 2016). M67 is a cornerstone of stellar astrophysics, and it explains why they are missing in Fig. 20. is a calibrator of age determination via gyrochronology (Barnes The left figures associated with the thin disk show the same et al. 2016). Its turn-off mass is very close to the critical mass for main features typical of a young population as the local HRD the onset of core convection. For this reason, the cluster is es- of Fig. 6: young hot main-sequence stars are present (Fig. 21a), pecially interesting for this specific regime of stellar models and the secondary red clump as well as the AGB bump is visible their dependence on different parameters such as nuclear reac- (Fig. 20a), and the turn-off region is diffusely populated. The tion rate and solar abundances. The main-sequence termination middle figures associated with the thick disk show a more lo- presents a distinctive hook and a gap just above it. These fea- calised turn-off typical of an intermediate to old population. The tures are used to distinguish between diffusive and non-diffusive median locus of the main sequence is similar to the thin-disk evolutionary models. Atomic diffusion is very important for the selection. The right figures associated with the halo show an ex- morphology of isochrones in the vicinity of the turn-off. The tended horizontal branch, typical of old metal-poor populations, hook feature traces the rapid contraction phase that occurs at but also two very distinct main sequences and turn-offs. We note central H exhaustion in those stars that have convective cores the presence of the halo white dwarfs. during their main-sequence phase. This hook is located at some- We study the kinematic selection associated with the halo what higher luminosities and cooler temperatures when diffusive in Fig. 22 in more detail. The two main-sequence turn-offs are processes are included (Michaud et al. 2004). Gaia photometry shifted by ∼0.1 mag in colour. The red main-sequence turn-off and parallax place the location of these features very precisely is shifted by ∼0.05 mag from the thick-disk kinematic selec- in the HRD. PARSEC isochrones, including overshoot and dif- tion main sequence (green line in Fig. 22a). Comparison with fusion, reproduce the main-sequence slope and termination point isochrones clearly identifies the distinct main sequences as be- reasonably well, although additional overshoot calibration might ing driven by a metallicity difference of about 1 dex. To further be necessary. A population of blue stragglers, a few yellow gi- confirm this, we cross-matched our selection with the APOGEE ants, and two sub-subgiants are clearly visible among the mem- DR14 catalogue (Holtzman et al. 2015) using their 2MASS ID bers. The binary star sequence in M67 is clearly defined as well. and the 2MASS cross-match provided in the Gaia archive (Mar- NGC 2447 is a younger object with an age of 0.55 Gyr and rese et al. 2018). There are 184 stars in common, 1168 if we relax almost solar metallicity. Previous photometry is relatively poor the low-extinction criteria that mostly confine our HRD selec- (Clariá et al. 2005). In Gaia DR2, photometry and membership tion to the galactic poles. The metallicity distribution is indeed of the cluster stand out very clearly. PARSEC isochrones repro- double-peaked, with peak metallicities of -1.3 and -0.5 dex. We duce the main sequence very well, while the red clump colour is superimpose in Fig. 22 the corresponding PARSEC isochrones slightly redder. using the Salaris et al. (1993) formula for the mean α enhance- Figure 19 presents the HRD of the globular cluster 47 Tuc ment of 0.23 for [M/H]=-1.3 and -0.5 and ages of 13 and 11 Gyr, (see Table 3), which is one prominent example of multiple popu- respectively. While the extent of the horizontal branch does not lations in globular clusters. Hubble Space Telescope (HST) pho- correspond to the isochrones used here, it can be compared to the tometry in the blue passbands has revealed a double main se- empirical horizontal branches of the globular clusters presented quence (Milone et al. 2012) and distinct subgiant branches (An- in Fig. 11. derson et al. 2009). These components are not visible in the high- This bimodal metallicity distribution in the kinematic selec- accuracy Gaia photometry, since bluer colours would be neces- tion of the halo may recall the globular cluster bimodal metallic- sary. 47 Tuc has a relatively high average metallicity of [Fe/H]=- ity distribution with the same peaks at [Fe/H]∼-0.5 and [Fe/H]∼- 0.72. We fit it with PARSEC isochrones with Z = 0.0056, 1.5 (e.g. Zinn 1985), the more metal-rich part being associated Y = 0.25. Since no alpha-enhanced tracks are available in the with the thick disk and bulge. We verified with the globular clus- PARSEC data set, we use the Salaris et al. (1993) relation to ter kinematics provided in Gaia Collaboration et al. (2018c) that account for the enhancement. 80% of these globular clusters indeed fall into our halo kine- matic selection, independently of their metallicity. The -0.5 dex peak also recalls the bulge metal-poor component (e.g. Hill et al. 7. Variation of the HRD with kinematics 2011). However, it seems to be different from the double halo Thin disk, thick disk, and halo have different age and metal- found at larger distances (Carollo et al. 2007; de Jong et al. licity distributions as well as kinematics. The Gaia HRD 2010): while their inner-halo component at ∼-1.6 could corre- is therefore expected to vary with the kinematics properties. spond to our metal-poor component, their metal-poor component For stars with radial velocities, we apply classical cuts to is at metallicity ∼-2.2 and is found in the outer Galaxy. This du- −1 ality in the metallicity distribution of the kinematically selected broadly kinematically select thin-disk (Vtot<50 km s ), thick- −1 −1 halo stars has also been found using TGAS data with RAVE and disk (70
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−1 −1 Fig. 20. Gaia HRDs with kinematic selections based on the total velocity: a) Vtot<50 km s (275,595 stars), b) 70
−1 −1 Fig. 21. Gaia HRDs with kinematic selections based on the tangential velocity: a) VT<40 km s (1,893,677 stars), b) 60
Article number, page 17 of 28 A&A proofs: manuscript no. DR2HRD
ipating in the Gaia MultiLateral Agreement (MLA). The Gaia mission web- site is https://www.cosmos.esa.int/gaia. The Gaia archive website is https://archives.esac.esa.int/gaia. The Gaia mission and data pro- cessing have financially been supported by, in alphabetical order by coun- try: the Algerian Centre de Recherche en Astronomie, Astrophysique et Géo- physique of Bouzareah Observatory; the Austrian Fonds zur Förderung der wis- senschaftlichen Forschung (FWF) Hertha Firnberg Programme through grants T359, P20046, and P23737; the BELgian federal Science Policy Office (BEL- SPO) through various PROgramme de Développement d’Expériences scien- tifiques (PRODEX) grants and the Polish Academy of Sciences - Fonds Weten- schappelijk Onderzoek through grant VS.091.16N; the Brazil-France exchange programmes Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Coordenação de Aperfeicoamento de Pessoal de Nível Superior (CAPES) - Comité Français d’Evaluation de la Coopération Universitaire et Scientifique avec le Brésil (COFECUB); the Chilean Dirección de Gestión de la Investigación (DGI) at the University of Antofagasta and the Comité Mixto ESO-Chile; the National Science Foundation of China (NSFC) through grants 11573054 and 11703065; the Czech-Republic Ministry of Education, Youth, and Sports through grant LG 15010, the Czech Space Office through ESA PECS contract 98058, and Charles University Prague through grant PRIMUS/SCI/17; the Danish Min- istry of Science; the Estonian Ministry of Education and Research through grant IUT40-1; the European Commission’s Sixth Framework Programme through the European Leadership in Space Astrometry (https://www.cosmos.esa. int/web/gaia/elsa-rtn-programmeELSA) Marie Curie Research Training Network (MRTN-CT-2006-033481), through Marie Curie project PIOF-GA- 2009-255267 (Space AsteroSeismology & RR Lyrae stars, SAS-RRL), and through a Marie Curie Transfer-of-Knowledge (ToK) fellowship (MTKD-CT- 2004-014188); the European Commission’s Seventh Framework Programme through grant FP7-606740 (FP7-SPACE-2013-1) for the Gaia European Net- work for Improved data User Services (GENIUS) and through grant 264895 for the Gaia Research for European Astronomy Training (https://www.cosmos. esa.int/web/gaia/great-programmeGREAT-ITN) network; the European Research Council (ERC) through grants 320360 and 647208 and through the European Union’s Horizon 2020 research and innovation programme through grants 670519 (Mixing and Angular Momentum tranSport of massIvE stars – MAMSIE) and 687378 (Small Bodies: Near and Far); the European Science Foundation (ESF), in the framework of the Gaia Research for European As- tronomy Training Research Network Programme (https://www.cosmos.esa. int/web/gaia/great-programmeGREAT-ESF); the European Space Agency (ESA) in the framework of the Gaia project, through the Plan for European Co- −1 Fig. 22. a) Same as Fig. 21c (kinematic selection VT>200 km s ) over- operating States (PECS) programme through grants for Slovenia, through con- laid with PARSEC isochrones for [M/H]= −1.3, age= 13 Gyr (blue), tracts C98090 and 4000106398/12/NL/KML for Hungary, and through contract and [M/H]= −0.5, age= 11 Gyr (magenta) and [α/Fe]= 0.23; green 4000115263/15/NL/IB for Germany; the European Union (EU) through a Eu- line: median spline fit to the main sequence of the thick-disk kinematic ropean Regional Development Fund (ERDF) for Galicia, Spain; the Academy selection (Fig. 21b). b) [α/Fe] vs. [Fe/H] of the corresponding APOGEE of Finland and the Magnus Ehrnrooth Foundation; the French Centre National de la Recherche Scientifique (CNRS) through action ’Défi MASTODONS’, stars without extinction criterion applied. c) Density distribution of the the Centre National d’Etudes Spatiales (CNES), the L’Agence Nationale de la tangential velocity VT on the blue and red sides of a median spline Recherche (ANR) ’Investissements d’avenir’ Initiatives D’EXcellence (IDEX) main-sequence fit. programme Paris Sciences et Lettres (PSL∗) through grant ANR-10-IDEX- 0001-02, the ANR ’Défi de tous les savoirs’ (DS10) programme through grant ANR-15-CE31-0007 for project ’Modelling the Milky Way in the Gaia era’ are observed for the faintest brown dwarfs, the white dwarf hy- (MOD4Gaia), the Région Aquitaine, the Université de Bordeaux, and the Uti- drogen/helium split, or the very fine structures of the open cluster nam Institute of the Université de Franche-Comté, supported by the Région de Franche-Comté and the Institut des Sciences de l’Univers (INSU); the German main sequences, for example, are expected to bring new insight Aerospace Agency (Deutsches Zentrum für Luft- und Raumfahrt e.V., DLR) into stellar physics. through grants 50QG0501, 50QG0601, 50QG0602, 50QG0701, 50QG0901, Numerous studies by the community are expected on the 50QG1001, 50QG1101, 50QG1401, 50QG1402, 50QG1403, and 50QG1404 Gaia HRD. For example, rare stages of evolution will be ex- and the Centre for Information Services and High Performance Computing (ZIH) at the Technische Universität (TU) Dresden for generous allocations of tracted from the archive, together with more clusters, and de- computer time; the Hungarian Academy of Sciences through the Lendület Pro- tailed comparisons with different stellar evolution models will gramme LP2014-17 and the János Bolyai Research Scholarship (L. Molnár be made. The completeness of the data is a difficult question that and E. Plachy) and the Hungarian National Research, Development, and Inno- we did not discuss here, but that will be studied by the commu- vation Office through grants NKFIH K-115709, PD-116175, and PD-121203; the Science Foundation Ireland (SFI) through a Royal Society - SFI Uni- nity as it is a very important issue, in particular for determining versity Research Fellowship (M. Fraser); the Israel Science Foundation (ISF) the local volume density and all the studies of the initial mass through grant 848/16; the Agenzia Spaziale Italiana (ASI) through contracts function and stellar evolution lifetimes. I/037/08/0, I/058/10/0, 2014-025-R.0, and 2014-025-R.1.2015 to the Italian The next Gaia release, DR3, will again be a new step for Istituto Nazionale di Astrofisica (INAF), contract 2014-049-R.0/1/2 to INAF dedicated to the Space Science Data Centre (SSDC, formerly known as the stellar studies. This will be achieved not only by the increase ASI Sciece Data Centre, ASDC), and contracts I/008/10/0, 2013/030/I.0, 2013- in completeness, precision, and accuracy of the data, but also 030-I.0.1-2015, and 2016-17-I.0 to the Aerospace Logistics Technology En- by the additional spectrophotometry and spectroscopy, together gineering Company (ALTEC S.p.A.), and INAF; the Netherlands Organisa- with the binarity information that will be provided. tion for Scientific Research (NWO) through grant NWO-M-614.061.414 and through a VICI grant (A. Helmi) and the Netherlands Research School for Acknowledgements. We thank Pierre Bergeron for providing the WD tracks Astronomy (NOVA); the Polish National Science Centre through HARMO- in the Gaia DR2 passbands. This work presents results from the European NIA grant 2015/18/M/ST9/00544 and ETIUDA grants 2016/20/S/ST9/00162 Space Agency (ESA) space mission Gaia. Gaia data are being processed by and 2016/20/T/ST9/00170; the Portugese Fundação para a Ciência e a Tec- the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the nologia (FCT) through grant SFRH/BPD/74697/2010; the Strategic Programmes DPAC is provided by national institutions, in particular the institutions partic- UID/FIS/00099/2013 for CENTRA and UID/EEA/00066/2013 for UNINOVA;
Article number, page 18 of 28 Gaia Collaboration et al.: Gaia Data Release 2: Observational Hertzsprung-Russell diagrams the Slovenian Research Agency through grant P1-0188; the Spanish Ministry Dufour, P., Blouin, S., Coutu, S., et al. 2017, in Astronomical Society of the of Economy (MINECO/FEDER, UE) through grants ESP2014-55996-C2-1-R, Pacific Conference Series, Vol. 509, 20th European White Dwarf Workshop, ESP2014-55996-C2-2-R, ESP2016-80079-C2-1-R, and ESP2016-80079-C2-2- ed. P.-E. Tremblay, B. Gaensicke, & T. Marsh, 3 R, the Spanish Ministerio de Economía, Industria y Competitividad through Evans, D., Riello, M., De Angeli, F., et al. 2018, A&A, this volume, DPACP-40: grant AyA2014-55216, the Spanish Ministerio de Educación, Cultura y De- photometric validation porte (MECD) through grant FPU16/03827, the Institute of Cosmos Sciences Faherty, J. K., Burgasser, A. J., Cruz, K. L., et al. 2009, AJ, 137, 1 University of Barcelona (ICCUB, Unidad de Excelencia ’María de Maeztu’) Fitzpatrick, E. L. & Massa, D. 2007, ApJ, 663, 320 through grant MDM-2014-0369, the Xunta de Galicia and the Centros Singu- Ford, A., Jeffries, R. D., & Smalley, B. 2005, MNRAS, 364, 272 lares de Investigación de Galicia for the period 2016-2019 through the Cen- Gaia Collaboration, Brown, A., Vallenari, A., et al. 2018a, A&A, this volume, tro de Investigación en Tecnologías de la Información y las Comunicaciones DPACP-36: DR2 overview (CITIC), the Red Española de Supercomputación (RES) computer resources at Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2016, A&A, 595, A2 MareNostrum, and the Barcelona Supercomputing Centre - Centro Nacional de Gaia Collaboration, Eyer, L., & et al. 2018b, A&A, this volume, DPACP-35: Supercomputación (BSC-CNS) through activities AECT-2016-1-0006, AECT- variable stars in the CMD 2016-2-0013, AECT-2016-3-0011, and AECT-2017-1-0020; the Swedish Na- Gaia Collaboration, Helmi, A., van Leeuwen, F., et al. 2018c, A&A, this volume, tional Space Board (SNSB/Rymdstyrelsen); the Swiss State Secretariat for Ed- DPACP-34: kinematics of globular clusters and dwarf galaxies ucation, Research, and Innovation through the ESA PRODEX programme, the Gaia Collaboration, Katz, D., Antoja, T., Romero-Gómez, M., & et al. 2018d, Mesures d’Accompagnement, the Swiss Activités Nationales Complémentaires, A&A, this volume, DPACP-33: Mapping the Milky Way kinematic structure and the Swiss National Science Foundation; the United Kingdom Rutherford Ap- Gaia Collaboration, van Leeuwen, F., Vallenari, A., et al. 2017, A&A, 601, A19 pleton Laboratory, the United Kingdom Science and Technology Facilities Coun- Gallart, C. 1998, ApJ, 495, L43 cil (STFC) through grant ST/L006553/1, the United Kingdom Space Agency Girardi, L. 1999, MNRAS, 308, 818 (UKSA) through grant ST/N000641/1 and ST/N001117/1, as well as a Particle Girardi, L. 2016, ARA&A, 54, 95 Physics and Astronomy Research Council Grant PP/C503703/1. 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Arquitectura de Computadores y Automática, Facultad de In- formática, Universidad Complutense de Madrid, C/ Prof. José Gar- cía Santesmases s/n, 28040 Madrid, Spain 129 H H Wills Physics Laboratory, University of Bristol, Tyndall Av- enue, Bristol BS8 1TL, United Kingdom 130 Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capita 2-4, 08034 Barcelona, Spain 131 Applied Physics Department, Universidade de Vigo, 36310 Vigo, Spain 132 Stellar Astrophysics Centre, Aarhus University, Department of Physics and Astronomy, 120 Ny Munkegade, Building 1520, DK- 8000 Aarhus C, Denmark 133 Argelander-Institut fü Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany 134 Research School of Astronomy and Astrophysics, Australian Na- tional University, Canberra, ACT 2611 Australia 135 Sorbonne Universités, UPMC Univ. Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd. Arago, 75014 Paris, France 136 Department of Geosciences, Tel Aviv University, Tel Aviv 6997801, Israel Gaia Collaboration et al.: Gaia Data Release 2: Observational Hertzsprung-Russell diagrams
Table A.1. Membership data for the open clusters. Only the first three lines with data for members of the Praesepe cluster are presented here. For the more distant clusters, the last two columns are not included. The astrometric and photometric extra filters presented in Sect. 2.1 and used in the figures of this paper are not applied in this table. The full table will be available in electronic form at the CDS.
DR2 SourceId cluster α δ $d σ$d (degr) (degr) mas mas 685747814353991296 Praesepe 133.15933 21.15502 5.645 0.033 685805259540481664 Praesepe 133.57003 21.73443 4.798 0.100 665141141087298688 Praesepe 130.22501 21.75663 5.630 0.020 ......
Fig. A.2. Surface-density profile for the Pleiades cluster, based on 1332 identified cluster members.
Nine clusters within 250 pc from the Sun were analysed as nearby clusters. The analysis is iterative, and consists of two ele- ments: 1. determinations of the space velocity vector at the clus- Fig. A.1. Maximum radius in degrees in DR2 for the 46 open clusters as ter centre, and 2. determination of the cluster centre. A first se- a function of parallax. The two diagonal lines represent maximum radii lection is made of stars contained in a sphere with a radius of of 10 (bottom) and 20 (top) pc. around 15 pc around the assumed centre of the cluster. A sum- mary of the observed radii for the nearby and more distant clus- ters is shown in Fig. A.1. The radius can be adjusted based on the Appendix A: Open cluster membership and derived surface density distribution (Fig. A.2), where the outer- astrometric solutions most radius is set at the point beyond which the density of con- taminating field stars starts to dominate. The selected stars are Appendix A.1: Nearby clusters further filtered on their agreement between the observed proper The nearby clusters were analysed with the method described motion and the predicted projection of the assumed space motion and applied to the Hyades cluster in the first Gaia data release at the 3D position of the star, using the measured stellar parallax, (Gaia Collaboration et al. 2017). By combining the information and taking into account the uncertainties on the observed proper from the measured proper motions and parallaxes for individual motion and parallax. The solution for the space motion follows cluster members, it is possible to derive a higher precision mea- Eq. A13 in Gaia Collaboration et al. (2017). Although it is in surement for the relative parallax of these cluster members. The principle possible to solve also for the radial velocity using only proper motion observed for an individual cluster member repre- the astrometric data, this effectively only works for the Hyades sents the local projection on the sky of the baricentric velocity cluster. Instead, a single equation for the observed radial veloc- of the cluster. It is therefore affected by the angular separation ity of the cluster was added, where the observed radial velocity on the sky of the member star from the projection of the clus- is based on the weighted mean of the Gaia radial velocities of ter centre and the baricentric distance of the star, again relative cluster members for which these data are available. to that of the cluster centre. Similarly, the measured parallax for To stabilise the solution, it is important to align the coordi- the star can be significantly different from the mean parallax of nate system with the line of sight towards the cluster centre, min- the cluster. imising the mixing of the contributions from the proper motions The primary aim of the present paper is to provide high- and the additional information from the radial velocity. The solu- precision HRDs, for which these accurate relative parallaxes tion for the space motion does provide an estimate of the radial contribute important information by reducing the actual differ- velocity component, but except for the Hyades and Coma Ber, ential distance modulus variations of cluster members. The ef- this is largely dominated by the radial velocity value and its ac- fectiveness of this procedure is limited by the amplitude of the curacy that is used as input to the solution. Small differences are proper motion of the cluster centre and the ratio of the diameter therefore seen between the radial velocities as presented in Ta- over the distance of the cluster. The standard uncertainties in the ble A.2 (as directly derived from the Gaia spectroscopic data) individual parallaxes and proper motions of the cluster members and in Table A.3 (the summary data for the nine clusters in this in the second Gaia data release allow for this procedure to be selection), where the astrometric information on the radial ve- applied for clusters within 250 pc. Table A.1 shows an example locity is also taken into account. Figure A.3 shows an example of an extract from the cluster member files produced for each of of the level of agreement between the differential parallax and the nine clusters treated in this way. proper motion values in the cluster IC 2602. In Fig. A.4 we also
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Appendix A.2: More distant open clusters For the more distant clusters, a selection was made of 37 rela- tively rich clusters, generally only little reddened, and as far as possible, covering a spread in ages and chemical composition (Fig. A.6). These clusters were all analysed in a combined solu- tion of the mean parallax and proper motion from the observed astrometric data of the member stars. This is an iterative proce- dure, where cluster membership determination is based on the solution for the astrometric parameters of the cluster. The com- bined solution for the astrometric parameters of a cluster takes into account noise contributions from three sources: 1. the covariance matrix of the astrometric solution for each star; 2. the internal velocity dispersion of the cluster, affecting the dispersion of the proper motions; 3. the effect of the cluster size relative to its distance, which (a) is reflected in a dispersion on the parallaxes of the cluster members; (b) is reflected in a dispersion in proper motions in the direc- tion of, and scaled by, the cluster proper motion. When we assume that the velocity distribution is isotropic within the measurement accuracy, then the second of these noise Fig. A.3. Comparison between the directly measured parallaxes and the parallaxes obtained by including the relative proper motion data, for the contributions will be diagonal. The first and third may also con- cluster IC 2602. The clear linear relation shows the good agreement be- tain significant off-diagonal elements. Given a cluster parallax of tween proper motion and parallax offsets from the mean cluster values. $c , a cluster proper motion of (µα,c, µδ,c), and an average relative dispersion in the parallaxes of the cluster stars of σ$/$ = σR/R Table A.2. Mean radial velocity values as derived from the Gaia spec- (where R is the distance to the cluster centre), the contribution troscopic data for nearby clusters. to the dispersion in the proper motions of the cluster stars scales with the relative dispersion of the parallaxes and the proper mo- tions of the cluster: Name Vrad σ(Vrad) uwsd Nobs Hyades 39.87 0.05 2.28 150 σµ = |µ | × σ /$ (A.1) ComaBer 0.21 0.13 2.15 43 α,s α,c $ Pleiades 5.54 0.10 2.00 195 σµδ,s = |µδ,c| × σ$/$. (A.2) IC2391 15.00 0.24 1.19 35 IC2602 17.62 0.22 1.24 36 For most of the clusters with distances beyond 250 pc, this con- alphaPer -0.32 0.17 1.38 71 tribution will be small to very small relative to other contribu- Praesepe 34.84 0.07 1.45 176 tions. Figure A.7 shows the overall relation between parallaxe NGC2451A 23.08 0.34 1.32 31 and proper motion amplitudes for the selection of clusters we Blanco1 6.01 0.15 1.03 51 used. The contributions are summed into a single noise matrix, of Notes. Columns: 1. Cluster name; 2: weighted-mean radial velocity in which an upper-triangular square root is used to normalise the km s−1; 3. standard uncertainty on radial velocity; 4. unit-weight stan- observation equations that describe the cluster proper motion dard deviation of mean velocity solution; and 5. number of observations and parallax as a function of the observed proper motions and in mean velocity solution. parallaxes of the individual cluster members. Table A.4 presents an overview of the astrometric solutions for 37 open clusters, with mean radial velocities when available show an example of the 3D distribution maps for this cluster; in the Gaia data. We note that some clusters are not included maps like this were prepared for all nearby clusters. in Table 2 because their colour-magnitude diagrams are too dis- Next to the astrometric data, the second Gaia data release turbed by interstellar extinction (see an illustration of the dif- also presents radial velocity measurements for a magnitude- ferential extinction effect in Fig. A.8). The proper motions are limited sample. The radial velocities were compared with the compared with those presented by Loktin & Beshenov (2003) projection of the cluster space velocity at the position of each in Fig. A.9, and they agree well overall, but there is also an in- star for which these data are available. This is particularly rele- dication that errors on the data presented in Loktin & Beshenov vant for stars in the Hyades cluster, where the projection effects (2003) are underestimated. In the same figure the comparison be- of the radial velocity can be of the order of several km s−1. Ta- tween the parallaxes as derived from the DR2 data and parallax ble A.2 presents the results for the nine nearby clusters. values derived from photometric distances as (mostly) presented Figure A.5 shows the differences (observed − predicted, in Kharchenko et al. (2005) are shown, and again generally agree where the predicted value is based on the local projection of well (see also the validation with more clusters in Arenou et al. the space velocity of the cluster) in the radial velocities for 191 2018). The systematic difference of 0.029 mas, which can be ob- stars in the Hyades cluster. Only stars for which the colour index served for globular clusters (Gaia Collaboration et al. 2018c), is GBP − GRP is greater than 0.4 mag were used. The results for all too small to be noticed here (Fig. A.10), but the calibration noise 9 nearby clusters are shown in Table A.2. on the DR2 parallaxes (0.025 mas), obtained in the same study,
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Fig. A.4. Distribution of stars in IC 2602 in galactic rectangular coordinates, showing the flattening in the Z (galactic pole) direction.
Table A.3. Space velocity fitting results for nearby clusters
Name U’ V’ W’ cU0V0 cU0W0 uwsd αc $ Vrad µα∗ µδ ClustId σU’ σV’ σW’ cV0W0 σv Observ. δc σ$ σVrad σµα∗ σµδ km/s km/s km/s degr. mas km/s mas/yr mas/yr Hyades -6.059 45.691 5.544 0.33 0.35 0.67 97.5407 21.052 39.96 101.005 -28.490 C0424+157 0.031 0.069 0.025 0.93 0.40 515 6.8148 0.065 0.06 0.171 0.137 ComaBer -1.638 4.785 -3.528 0.35 -0.86 0.48 110.1896 11.640 -0.52 -12.111 -8.996 C1222+263 0.078 0.018 0.040 -0.39 0.40 153 -34.3206 0.034 0.07 0.048 0.121 Pleiades -1.311 21.390 -24.457 0.48 0.50 0.77 93.5183 7.364 5.65 19.997 -45.548 C0344+239 0.070 0.105 0.057 0.90 0.40 1326 -48.7831 0.005 0.09 0.127 0.101 Praesepe 0.339 49.097 1.200 -0.50 -0.60 0.76 89.5122 5.371 35.64 -36.047 -12.917 C0937+201 0.090 0.106 0.050 0.92 0.40 938 1.3517 0.003 0.10 0.110 0.066 alphaPer -5.110 24.183 -14.122 0.25 0.40 0.68 101.9183 5.718 -0.29 22.929 -25.556 C0318+484 0.053 0.067 0.097 0.59 0.40 740 -29.7555 0.005 0.08 0.071 0.095 IC2391 -0.751 28.459 -1.590 -0.20 0.38 0.68 91.6471 6.597 14.59 -24.927 23.256 C0838-528 0.054 0.062 0.105 -0.52 0.40 325 -3.4126 0.007 0.09 0.080 0.110 IC2602 -9.467 16.867 -12.377 -0.05 0.40 0.72 119.3285 6.571 17.43 -17.783 10.655 C1041-641 0.056 0.024 0.120 -0.16 0.40 492 -32.7371 0.007 0.11 0.040 0.098 Blanco1 6.176 21.150 -0.296 0.01 -0.86 0.65 73.6042 4.216 5.78 18.724 2.650 C0001-302 0.111 0.020 0.065 -0.02 0.40 489 -0.8388 0.003 0.10 0.017 0.070 NGC2451 5.806 32.440 -3.100 -0.24 0.34 0.68 79.8905 5.163 22.85 -21.063 15.378 C0743-378 0.048 0.095 0.084 -0.76 0.40 400 -5.4202 0.005 0.09 0.065 0.093
Notes. Columns: 1. Cluster identifiers; 2 to 4 U’, V’ and W’ velocity components in the equatorial system; 5. U’V’ error correlation (top) V’W’ error correlation (bottom); 6. U’W’ error correlation (top), applied internal velocity dispersion in km s−1 (bottom); 7. unit-weight standard deviation of solution (top), number of stars (bottom); 8. Coordinates of the convergent point; 9. parallax (mas); 10. radial velocity (km s−1); 11. proper motion in right ascension; and 12. proper motion in declination. is significantly larger than the standard uncertainties on the mean have reached the maximum radius for the cluster in this partic- cluster parallaxes and is therefore the main contributor to the un- ular data set and parameter space. It is well possible, however, certainties on the cluster parallaxes. In most cases, however, this that for a catalogue with higher accuracies on the astrometric amounts to less than 1% in error on the parallax, or 0.02 mag in parameters for the fainter stars in particular, this limit will be distance modulus. found still farther away from the cluster centre. Radial velocities The maximum radius for each cluster was determined from for the clusters, mostly as given in Kharchenko et al. (2005) or the contrast between the cluster and the field stars in the proper Conrad et al. (2014), were compared with the mean radial ve- motion and parallax domain. In practice, this means that the den- locities as derived from the Gaia DR2 data. A limited spectral sity of field stars for which the combined information on the par- range was used, for which there is clear consistency of the radial allax and proper motion, combined with uncertainties and error velocity measurements. The summary of the results is shown in correlations, leaves a significant possibility for a field star to be Fig. A.9 and generally agrees well (see also the validation with a cluster member. When the surface density of these field stars more clusters in Arenou et al. 2018). The largest discrepancies becomes similar to the surface density of the cluster stars, we
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Fig. A.5. Differences between the predicted and observed radial veloc- ities in the Hyades cluster as a function of G magnitude. are found for NGC 2516 (RAVE measurements in Conrad et al. 2014) and Trumpler 2 (Kharchenko et al. 2005).
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Fig. A.6. Distributions over age and composition for stars in the 32 open clusters selected for the composite HRD, including the nearby clusters.
Fig. A.7. Comparison between the parallaxes and proper motions for the 37 open clusters. The upper and lower diagonal lines represent tan- gential velocities of 40 and 5 km s−1 , respectively.
Fig. A.8. Colour-magnitude diagram of NGC 2477, with each star colour-coded by the value of integrated extinction in the catalogue of Schlegel et al. (1998).
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Fig. A.9. Comparisons with values quoted in literature (see text) for (top) proper motions in right ascension, proper motions in declination, (bottom) parallaxes, and radial velocities for 37 open clusters with distances beyond 250 pc.
Fig. A.10. Standard uncertainties on the mean cluster parallax deter- minations. The curves represent the 100 and 500 σ significance levels when only the standard uncertainties are considered.
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Table A.4. Overview of the results for open clusters with distances beyond 250 pc
Name α $ µα∗ µδ c12 c23 nMemb Vrad uwsd ◦ ClustId δ σ$ σµα∗ σµδ c13 r(max) st.dev. σ Obs. degr. mas mas/yr mas/yr km/s NGC0188 11.7494 0.5053 -2.3087 -0.9565 -0.04 -0.02 1181 -41.86 1.43 C0039+850 85.2395 0.0011 0.0035 0.0030 0.16 0.58 0.84 0.13 20 NGC0752 29.2054 2.2304 9.8092 -11.7637 0.02 -0.04 433 5.90 1.68 C0154+374 37.7454 0.0027 0.0191 0.0180 0.04 2.58 0.86 0.11 76 Stock2 33.8282 2.6367 15.8241 -13.7669 0.01 0.01 1742 8.58 1.45 C0211+590 59.5813 0.0009 0.0103 0.0104 -0.00 2.36 0.78 0.09 109 NGC0869 34.7391 0.3942 -0.6943 -1.0831 0.14 0.10 829 C0215+569 57.1339 0.0014 0.0038 0.0041 0.08 0.19 0.83 0 NGC0884 35.5430 0.3976 -0.6021 -1.0616 0.16 0.11 1077 -44.69 4.98 C0218+568 57.1591 0.0012 0.0035 0.0036 0.10 0.29 0.86 0.73 2 Trump02 39.1879 1.4316 1.5305 -5.3361 0.05 0.04 589 -4.06 0.75 C0233+557 55.8846 0.0023 0.0116 0.0117 0.01 1.21 0.90 0.09 4 NGC1039 40.5843 1.9536 0.7256 -5.7320 0.02 -0.02 764 -7.27 1.44 C0238+425 42.7027 0.0027 0.0109 0.0103 0.04 1.87 0.79 0.72 18 NGC1901 79.6838 2.3582 1.5953 12.6920 0.03 0.10 290 1.62 1.60 C0518-685 -68.1627 0.0031 0.0276 0.0277 -0.03 2.30 1.04 0.56 16 NGC2158 91.8751 0.1833 -0.1665 -1.9932 0.18 -0.19 3942 26.64 2.30 C0604+241 24.1163 0.0021 0.0035 0.0029 0.21 0.24 0.92 0.60 11 NGC2168 92.2688 1.1301 2.2784 -2.9336 0.08 -0.08 1794 -7.70 2.73 C0605+243 24.3148 0.0013 0.0052 0.0050 0.05 1.20 0.87 0.27 6 NGC2232 96.9973 3.0710 -4.7737 -1.9014 0.04 -0.04 318 24.22 0.96 C0624-047 -4.7929 0.0033 0.0185 0.0181 0.04 2.76 0.78 0.44 9 Trump10 131.8982 2.2637 -12.3536 6.5309 0.02 0.00 947 21.97 1.00 C0646-423 -42.5192 0.0014 0.0102 0.0104 -0.01 1.69 0.82 0.31 28 NGC2323 105.7245 1.0012 -0.7977 -0.6540 0.06 -0.03 382 11.55 C0700-082 -8.3586 0.0017 0.0063 0.0063 0.00 0.73 0.87 1 NGC2360 109.4452 0.9018 0.3853 5.5893 0.07 -0.02 1037 28.02 1.74 C0715-155 -15.6317 0.0012 0.0048 0.0048 -0.05 0.74 0.79 0.19 15 Coll140 111.0308 2.5685 -8.1285 4.7105 0.02 0.02 332 18.53 1.75 C0722-321 -32.1113 0.0025 0.0215 0.0220 -0.01 2.69 0.81 1.85 5 NGC2423 114.2904 1.0438 -0.7343 -3.6333 0.09 -0.00 694 18.50 2.04 C0734-137 -13.8348 0.0017 0.0070 0.0069 -0.04 1.04 0.81 0.17 19 NGC2422 114.1463 2.0690 -7.0200 0.9592 0.05 0.01 907 36.21 1.42 C0734-143 -14.4844 0.0014 0.0098 0.0099 -0.02 1.45 0.74 0.57 30 NGC2437 115.4358 0.6005 -3.8232 0.3729 0.11 0.01 3032 37.34 C0739-147 -14.8506 0.0009 0.0031 0.0031 -0.06 0.74 0.83 1 NGC2447 116.1262 0.9603 -3.5680 5.0434 0.03 0.01 926 22.37 3.01 C0742-237 -23.8567 0.0013 0.0056 0.0057 -0.01 1.00 0.80 0.26 11 NGC2516 119.5469 2.4118 -4.6579 11.1517 0.02 -0.00 2518 23.78 1.39 C0757-607 -60.7749 0.0006 0.0075 0.0075 -0.01 2.54 0.83 0.11 156 NGC2547 122.5654 2.5438 -8.5999 4.2542 0.02 0.00 644 15.46 2.47 C0809-491 -49.0498 0.0015 0.0148 0.0148 -0.00 2.79 0.78 0.83 22 NGC2548 123.3834 1.2897 -1.3302 1.0164 0.13 0.00 509 8.83 1.77 C0811-056 -5.7363 0.0024 0.0095 0.0093 -0.03 0.56 0.80 0.27 8 NGC2682 132.8476 1.1325 -10.9737 -2.9396 0.08 -0.00 1520 34.05 1.94 C0847+120 11.8369 0.0011 0.0064 0.0063 -0.01 1.06 0.76 0.10 66 NGC3228 155.3791 2.0323 -14.8800 -0.6498 0.03 0.03 222 C1019-514 -51.7693 0.0029 0.0220 0.0220 -0.01 2.27 0.81 0 NGC3532 166.3975 2.0659 -10.3790 5.1958 0.03 -0.02 1879 4.85 2.24 C1104-584 -58.7335 0.0007 0.0079 0.0079 0.01 2.31 0.79 0.13 143 NGC6025 240.7714 1.2646 -2.8846 -3.0222 -0.02 0.01 452 -7.66 C1559-603 -60.4562 0.0015 0.0100 0.0099 0.03 0.94 0.75 1 NGC6281 256.1638 1.8716 -1.8764 -3.9506 -0.03 0.05 573 -5.02 2.17 C1701-378 -37.9180 0.0019 0.0144 0.0136 0.05 1.19 0.80 0.20 21 IC4651 261.2035 1.0542 -2.4051 -5.0280 -0.07 0.10 960 -30.32 3.41 C1720-499 -49.9185 0.0014 0.0061 0.0060 0.06 0.76 0.80 0.19 56 NGC6405 265.1220 2.1626 -1.3662 -5.8063 -0.04 0.11 967 -9.20 5.39 Continued on next page
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Table A.4 – continued from previous page Name α $ µα∗ µδ c12 c23 nMemb Vrad uwsd ◦ ClustId δ σ$ σµα∗ σµδ c13 r(max) st.dev. σ Obs. degr. mas mas/yr mas/yr km/s C1736-321 -32.4135 0.0021 0.0140 0.0132 0.04 1.46 0.82 0.77 17 IC4665 266.4978 2.8918 -0.8993 -8.5114 -0.02 0.04 174 -11.26 1.86 C1743+057 5.5653 0.0034 0.0347 0.0345 0.02 2.39 0.75 2.12 6 NGC6475 268.2736 3.5704 3.0722 -5.3157 -0.02 0.04 1140 -14.84 2.63 C1750-348 -34.6639 0.0016 0.0185 0.0184 0.02 3.86 0.82 0.17 113 NGC6633 276.8737 2.5232 1.1584 -1.7371 -0.03 0.09 321 -28.59 1.83 C1825+065 6.6081 0.0023 0.0199 0.0200 0.01 1.99 0.84 0.14 28 IC4725 277.9462 1.5043 -1.7201 -6.1010 -0.07 0.09 755 C1828-192 -19.1058 0.0019 0.0091 0.0091 0.04 1.53 0.89 0 IC4756 279.6698 2.0943 1.2574 -4.9145 -0.04 0.06 543 -24.72 2.76 C1836+054 5.3836 0.0018 0.0134 0.0134 0.02 2.05 0.84 0.17 38 NGC6774 289.1055 3.2516 -0.9733 -26.6464 -0.03 0.11 234 41.79 3.36 C1913-163 -16.3901 0.0038 0.0367 0.0383 0.00 3.74 1.00 0.15 62 NGC6793 290.7795 1.6672 3.8120 3.5622 -0.03 0.06 465 -10.85 C1921+220 22.1400 0.0021 0.0131 0.0136 -0.02 1.47 0.81 1 NGC7092 322.4220 3.3373 -7.3569 -19.5993 -0.02 -0.00 433 -5.07 0.95 C2130+482 48.1315 0.0024 0.0256 0.0260 -0.00 3.72 0.86 0.21 21
Appendix B: Gaia archive query The Gaia archive6 query corresponding to the filters described in Sect. 2.1 is the following (selecting here the first five stars):
SELECT TOP 5 phot_g_mean_mag+5*log10(parallax)-10 AS mg, bp_rp FROM gaiadr2.gaia_source WHERE parallax_over_error > 10 AND phot_g_mean_flux_over_error>50 AND phot_rp_mean_flux_over_error>20 AND phot_bp_mean_flux_over_error>20 AND phot_bp_rp_excess_factor < 1.3+0.06*power(phot_bp_mean_mag-phot_rp_mean_mag,2) AND phot_bp_rp_excess_factor > 1.0+0.015*power(phot_bp_mean_mag-phot_rp_mean_mag,2) AND visibility_periods_used>8 AND astrometric_chi2_al/(astrometric_n_good_obs_al-5)<1.44*greatest(1,exp(-0.4*(phot_g_mean_mag-19.5)))
6 https://gea.esac.esa.int/archive/
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